Complete failure to install

[TheDreadPirate]

Because Plex required you to login with their servers, that also allowed them to do some networking wizardry to have their infrastructure act as a reverse proxy.  Obviously Jellyfin can't do that since the point is to self host everything.

You have two paths.  The simple path and the harder, but right, path.

Simple:

1) Sign up for a free DDNS domain (NoIP or DuckDNS)
2) Install the IP updater app from the DDNS provider on your PC.  This will automatically update your IP address for your DDNS address if/when it changes.
3) Setup port forwarding on your router for port 8096 and direct it to your PC running Jellyfin.

Done

Hard:

1) Do steps 1 and 2 from Simple.
2) Setup port forwarding on your router for port 80 and 443
3) Setup Caddy to act as your reverse proxy.  https://jellyfin.org/docs/general/networking/caddy/
4) Get HTTPS certificates from Let's Encrypt

One of our community members made a youtube video for this.  https://www.youtube.com/watch?v=dbmgOxPwQA0